The present invention is related to electrical measurement in general, particularly for the characterization of semiconductor devices.
In electronics, there is often a need to determine the time constant of the transient response of a circuit or a device. For device characterization, the transient response may be sensitive to temperature. One application of such a measurement is to determine the emission rate and the activation energy of traps in semiconductors. The presence of traps can affect the performance of a semiconductor device because they create recombination centers which can shorten the diffusion length and the lifetime of the carriers of the semiconductor. A short lifetime decreases the efficiency of a solar cell and the gain of a transistor.
One method of determining the trap emission rate and its activation energy is the deep level transient spectroscopy technique, as described by D. V. Lang, in the Journal of Applied Physics, P. 3014, 1974. In this method, an electrical pulse is applied to a p-n junction of semiconductor material, the trap emission rate and trap activation energy of which are to be determined. The polarity of the pulse is such that a depletion layer is first created and then collapsed. The excess charge introduced by the pulse will be captured by those traps which are now in the neutral region. The time it takes for the carriers to escape after the pulse disappears is a measure of the trap emission rate. This is indicated by the time constant of the transient response.
In measuring the time constant of the transient response, one may encounter noise problems. If the junction capacitance area is small, the signal to noise ratio of the transient response may be low. Special techniques have been developed to maximize the signal-to-noise ratio. One popular method is the "double box-car" method as described by Lang in the Journal of Applied Physics, P. 3023, 1974. In the double box method, the amplitude difference of the transient response at a fixed time interval is measured. The pulse repetition rate is fixed, but the temperature of the sample is varied. At a certain temperature, the amplitude difference is at a maximum. This maximum amplitude is an indication of emission rate. By changing the sampling time, the maximum response occurs at different temperatures. From a plot of logarithmic time constant versus temperature, the activation energy of the trap level can be determined by the slope.
Another method of determining the time constant is to use a lock-in amplifier instead of a double box-car. Maximum output occurs at a certain temperature when the lock-in amplifier operating frequency is correlated with the fundamental component of the transient response. This method was described by Kimerling in the Annual Review of Material Science, p. 417, 1977.
In either the double box-car or the lock-in amplifier method it is necessary to scan the temperature. The drawbacks of temperature scanning are that (a) it is time consuming, and (b) it is difficult to hold the test sample temperature constant while the temperature is scanned. There is a need for improvement.